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Concrete Slab Calculator — Yards, Bags & Cost
Enter your slab dimensions and get cubic yards, bag count, ready-mix trucks, and a cost estimate by region — instantly. No math needed.
Free Tool
Concrete Slab Calculator
Works for patios, garage floors, sidewalks, pool surrounds, and any rectangular slab. Results include 10% waste overage.
Results include 10% overage. Material costs from NRMCA 2026 regional averages. Installed costs include labor, forming, finishing. Always get 2–3 local quotes.
The Formula
How to Calculate Concrete for a Slab
The math is simple — but the details matter. Here's exactly what the calculator does and why.
Measure length and width in feet
For irregular shapes, break into rectangles and add results together. Use the Quantity field for multiple identical slabs (e.g. 4 sidewalk panels).
Enter thickness in inches
Most residential slabs are 4–6 inches. The calculator converts to feet automatically. See the thickness guide below to pick the right depth for your project.
Divide by 27
Cubic feet ÷ 27 = cubic yards. One cubic yard is the standard unit for ready-mix orders. The calculator does this automatically and adds 10% overage.
Choose bags or ready-mix
Under 1 yd³ — use bags. Over 1 yd³ — ready-mix is almost always cheaper. The cost breakdown shows both options side by side so you can decide instantly.
ACI 318 Standards
Concrete Slab Thickness Guide
Thickness is the most important variable in the calculation — and the most common mistake. Use this table to pick the right depth before you calculate.
| Slab Type | Recommended Thickness | Min. Thickness | Rebar? | Notes |
|---|---|---|---|---|
| Patio / Walkway | 4" | 3.5" | Optional | Foot traffic only. Wire mesh acceptable. |
| Garage floor | 4–6" | 4" | Recommended | Passenger vehicles. #3 rebar on 18" grid. |
| Driveway | 6" | 5" | Required | #4 rebar. Thicker for trucks or RVs. |
| Pool surround | 4" | 4" | Recommended | Slip-resistant finish. Slight drainage slope. |
| Shed / outbuilding | 4" | 3.5" | Optional | Thicker at perimeter if walls will bear on slab. |
| Barn / heavy equipment | 6–8" | 6" | Required | Fiber reinforcement or #4 rebar on 12" grid. |
| Commercial floor | 6–10" | 6" | Required | Engineer spec required. Post-tensioning common. |
Concrete Slab Cost by Size — Installed (2026)
| Slab Size | Sq Ft | Yd³ @ 4" | 80 lb bags | Installed cost (avg) |
|---|---|---|---|---|
| 10 × 10 ft | 100 | 1.37 | ~62 | $600 – $1,000 |
| 12 × 12 ft | 144 | 1.98 | ~89 | $864 – $1,440 |
| 16 × 16 ft | 256 | 3.51 | ~158 | $1,536 – $2,560 |
| 20 × 20 ft | 400 | 5.49 | ~247 | $2,400 – $4,000 |
| 24 × 24 ft | 576 | 7.91 | ~356 | $3,456 – $5,760 |
| 30 × 30 ft | 900 | 12.35 | ~556 | $5,400 – $9,000 |
| 40 × 60 ft | 2,400 | 32.9 | ~1,481 | $14,400 – $24,000 |
Concrete Mix for Slabs — Which PSI to Order
| PSI Rating | Use Case | Typical Mix | Notes |
|---|---|---|---|
| 2,500 PSI | Sidewalks, patios, light residential | 1:2:3 (cement:sand:aggregate) | Minimum for exterior slabs |
| 3,000 PSI | Driveways, garage floors, most residential | Standard ready-mix default | Most common order; good freeze-thaw resistance |
| 3,500 PSI | Driveways with heavy vehicles, barn floors | Higher cement content | Specify when ordering if needed |
| 4,000 PSI+ | Commercial, industrial, structural | Engineer-specified | Requires engineering approval in most jurisdictions |
Common Questions
Concrete Slab Calculator FAQ
Multiply length (ft) × width (ft) × thickness (ft) to get cubic feet, then divide by 27 to convert to cubic yards. For a 20×20 ft slab at 4 inches thick: 20 × 20 × 0.333 = 133.3 ft³ ÷ 27 = 4.94 yd³. Always add 10% overage, bringing the order to 5.43 yd³. Use the calculator above to skip the math entirely.
The ACI (American Concrete Institute) minimum for exterior residential slabs is 4 inches for patios and walkways, 6 inches for driveways and garage floors subject to vehicle traffic. Increase to 8 inches if you expect trucks, RVs, or heavy equipment. Thinner slabs crack under load and are not code-compliant in most jurisdictions.
A 20×20 ft (400 sq ft) concrete slab at 4 inches thick requires about 5.5 cubic yards of concrete. Installed cost in 2026 typically runs $2,400–$4,000 ($6–$10/sq ft) for a basic patio, or up to $6,000 for a reinforced garage floor or driveway at 6 inches. Material-only (ready-mix delivered) runs $650–$800 for this size in most regions.
A 10×10 ft slab at 4 inches deep = 1.37 cubic yards (with 10% overage). At 45 bags per cubic yard, you need approximately 62 bags of 80 lb concrete (Quikrete or Sakrete). At 60 bags per cubic yard for 60 lb bags, that's about 82 bags. For this size, bags are practical — a ready-mix truck has a minimum order that would leave you with significant leftover.
For patios and walkways with foot traffic only, wire mesh or fiber reinforcement is usually sufficient. For driveways, garage floors, and any slab bearing vehicle loads, #3 or #4 rebar on an 18-inch grid is required by most building codes. Rebar adds $0.50–$1.50/sq ft to the installed cost but significantly reduces cracking risk and extends slab life.
For most residential slabs, order 3,000 PSI — this is the standard ready-mix default and provides good freeze-thaw resistance. Use 2,500 PSI for light-duty interior slabs. Upgrade to 3,500 PSI for driveways subject to heavy vehicle loads, or in climates with frequent freeze-thaw cycles. Specify the PSI when calling your ready-mix supplier.
Thickness recommendations verified against ACI 318-19 and IRC Section R506. Cost data from NRMCA 2026 regional survey and RS Means construction cost data. Bag yield figures from Quikrete and Sakrete published product specifications. Updated June 2026.